In common radiology clinical environments the diagnostic radiologist seldom sees the patient in person. Examination procedures are performed by trained and qualified radiology technicians, or, in the case of ultrasound imaging, by sonographers. Examination results are communicated to the radiologist diagnostician by means of rapid access hard copy images on film. The film images are interpreted or read by being placed on a light box. The system is quick and cost-effective and conserves scarce resources since it allows both the radiologist and the diagnostician to work with increased efficiency. For such a system it is obvious that quickly available hard copies of very high image quality and high stability are critical.
For information which is presented in black-and-white or gray- scale form, technology to record and view diagnostic films has been highly developed over the years. The common x-ray film system is an example. Film image quality can be very good, having high spatial resolution and a wide gray scale. Access to the permanent record is quick, because developed and dry film images can be delivered within about two minutes after an examination is performed. The film is archivally permanent and cost is low.
In the last few years, however, there has been an increasing use of color in diagnostic images. In some imaging modalities color in the images serves to emphasize selected details or features, such as by using certain colors to designate densities or density gradients beyond preselected thresholds. This adds no new information but facilitates interpretation of information already contained in the image. However, in other areas, such as, for example, in diagnostic ultrasound, color is used to present new information which is not available in the black-and-white gray-scale image. This is done, for example, in color-flow or color-doppler images, wherein information about direction and velocity of blood flow is presented by color coded patches superimposed on the gray scale image everywhere the ultrasound slice intersects a blood vessel. The color-doppler imaging technique is gaining wide acceptance in certain diagnostic procedures.
However, hard copy color images are more time-consuming for development and delivery, and are more expensive and less convenient. Hard copy color images are not as archivally stable and often are not viewable by transmitted light, nor do they always provide true diagnostic image quality. There are of course a number of known methods for obtaining color images, but none has the combination of advantages provided by commonly used black-and-white film.
In conventional color transparency systems a silver halide photographic film, for example, a so called integral tri-pack of three layered light sensitive emulsions is used, where each layer is sensitive to one primary color. During exposure three color records are made as latent images in the three layers. During chemical processing after exposure the latent images are developed as silver images and then converted to dye images in the complementary colors. In this technique, as in most other color systems, the complete color pictorial image in viewable form is assembled on the material which forms the permanent record media.
Another example of color printing is thermal dye transfer. In this process three subtractive primary color dyes, each coated on its own piece of substrate, are successively brought into contact with a receiving substrate and the dyes are selectively transferred by sublimation, activated by a thermal scanning head which passes over the line of contact between the dye and the receiving substrate. After three passes the result again is a complete color pictorial image in viewable form assembled on the material which is the permanent record medium.
Yet another example of a color printing technique is the non-photographic use of ink jet printing. In this process multiple jets of variously colored ink are selectively squirted onto a substrate as it passes by. The result again is a complete color pictorial image in viewable form assembled on the material which is the record medium.
All of the known color hard copy systems employ separate color channels that are combined into a color image on the hard copy material itself. In the prior art the color image itself becomes the permanent pictorial record, with all of the inherent disadvantages of a permanent color record. The black-and-white record provides higher resolution, better gray scale capability, better temporal stability, and simpler and quicker film processing and handling by widely available standard equipment.
Thus it will be seen that the conventional black-and-white photography, which has long been a standard in the industry, has the many advantages of being quick, cheap, convenient, archivally stable, viewable by transmitted light and having high diagnostic image quality. In addition, the widespread and long-term use of black-and-white imaging has made the technique and the required processing equipment and its use well known and widely available. However, as mentioned above, the black-and-white technique lacks the ability to contain certain information that can be included in a multi-color image. Multi-color image techniques previously available, on the other hand, although capable of containing the additional information, lack the many advantages of the standard black-and-white processes.
Accordingly, it is an object of the present invention to provide a multi-color record that is readily viewable while avoiding or minimizing problems of prior art imaging systems.